The present invention relates to a control system having independent and cooperative control functions.
There exist control systems each controlling a variety of target systems. Where such target systems interact with one another, the system variables of these target systems can interfere with one another. That is, when a given target system is controlled and its system variable is increased or decreased accordingly, the system variable of another target system can be varied under the influence of the other system variable being controlled.
The above aspect of the multivariable control scheme is described illustratively in Japanese Patent Laid-Open No. 235898/1988 (from line 16 in the left-hand column through line 1 in the right-hand column, in the upper part of page 3).
The disclosure in Japanese Patent Laid-Open No. 235898/1988 involves having a plurality of independent controllers controlled by multivariable cooperative controllers. The cited examples of the independent controllers include a recirculation flow rate controller and a pressure controller. FIG. 5 of Japanese Patent Laid-Open No. 240897/1989 shows a plurality of local controllers (i.e., independent controllers) being controlled by multivariable cooperative controllers.
FIG. 4 of Japanese Patent Laid-Open No. 240897/1989 describes a setup involving two independent controllers and two cooperative controllers. In this setup, one cooperative controller outputting a signal to be added to a reference value u, of one independent controller inputs the output of the other independent controller; the other cooperative controller inputs the output of one independent controller and outputs a signal to be added to the reference value of the other independent controller.
The above and other similar conventional control systems have a problem. For example, if a cooperative controller fails, the failure affects the two independent controllers in their control operations. As a result, the control response of any of the target systems under control can deteriorate.
This holds true of what is disclosed in Japanese Patent Laid-Open No. 235898/1988 and in FIG. 5 of Japanese Patent Laid-Open No. 240897/1989. In these cases, a failure in a multivariable cooperative controller will likely affect the two independent controllers that receive the output of the failed cooperative controller.
In the setup of FIG. 4 in Japanese Patent Laid-Open No. 240897/1989, one failed cooperative controller will likely affect one of the independent controllers receiving the output of the failed unit, the other cooperative controller that receives the output of the independent controller receiving the output of the failed unit, and the other independent controller that receives the output of the other cooperative controller. In this manner, one failed cooperative controller can degrade the control response of a plurality of target systems under control.